I would try and make it more mushroom-like. It doesn't scream "mushroom!" when you first look at it. Instead it looks like two blobs.
The best way to do this is to look at lots of reference images of mushrooms and compare.
The cap of your mushroom looks fine, but it could be more round and plate-like, or spherical, depending upon the mushroom.
Also the stem is much too fat. Maybe thin it out and lengthen it a bit. Can't wait to see what you come up with!